Officials are trying to determine exactly what caused a house in Center City to explode while crews were investigating a reported carbon monoxide leak.

The incident happened in the 2300 block of Naudain Street.

Firefighters were called around 3:00 a.m. Thursday after residents reported the smell of gas.

"Upon arrival, they obtained a high level of carbon monoxide," said Deputy Commissioner Richard Dawson.

Residents were evacuated from surrounding homes and apartments as PGW workers tried to determine the source.

"We heard the fire trucks, heard the work going on, and smelled something that smelled like gas," said Nick Roussonou.

Firefighters spotted a fire burning in the basement of one rowhome then came the explosion that blew that home to pieces.

"A loud explosion! A firefighter was running by, he said he just got hit by a brick wall," said Rodney Dean.

The flames spread to the surrounding houses, burning about half of an adjacent apartment building before firefighters brought it under control around 5:20 a.m.

Mayor Michael Nutter came to see the explosion site later Thursday morning. The Red Cross is helping about 13 people who have been displaced.

"This is all going on as we have four shelters open for flooding so our volunteers have been running around, making sure everyone's safe. So far it's all going smoothly," said Dave Schrader, Red Cross.